The mammalian sirtuins (SIRT1-7) are NAD+-dependent deacetylase enzymes that regulate adaptations of cells to nutrient availability and are linked to the life-extending and health-providing benefits of low calorie diets. The distinct sirtuins play different roles in regulating cell physiology, and distinct isoforms play roles that depend on tissue. These enzymes regulate numerous processes important for adaptation to low calorie diets including: adipogenesis, adipolysis, mitochondrial biogenesis, insulin secretion, fatty acid oxidation and stress resistance. Importantly, sirtuins are generally upregulated in cell by low calorie conditions. The mechanisms by which sirtuins achieve their functions are still poorly understood although it is apparent that their catalytic activity, abundance and localization within cells are crucial to their biological functions. For example, sirtuins are naturally compartmentalized; SIRT1, SIRT6 and SIRT7 are nuclear, whereas SIRT3, SIRT4 and SIRT5 are mitochondrial. SIRT2 is cytosolic. In response to various physiologic conditions, sirtuins relocalize in cells to cause changes in cell biology. There are currently no tools that can define in a simultaneous way, catalytic activity, abundance and localization of sirtuins in cells. The development of such tools is highly challenging, but would certainly provide a powerful new approach to studying sirtuins if they could be developed. This grant proposal addresses that need by presenting a plan to develop isoform-specific probes for sirtuins that can be used to visualize activity, abundance and localization using click chemistry and optical microscopy methods. Prior work has established that small peptides containing thioacetyllysine residues are excellent general inhibitors of sirtuins. This inhibition uses the enzyme mechanism. These thiopeptides, if appropriately modified with alkynyl groups or other clickable groups, form stable thioimidate conjugates on sirtuins that can be crosslinked to other moieties by click chemistry. We have demonstrated that examples of these clickable thioacetyllysine peptides are cell permeable and form stable complexes to sirtuins in cells. By known methods, the conjugates formed on sirtuin active sites in cells are proposed to be clicked to dyes, thus allowing visualization of intracellular sirtuin activity, abundance and localization. To accomplish these goals we provide the following specific aims: 1) We propose to develop a set of isoform specific clickable cell permeable thioacetyllysine tripeptides that can be used to image sirtuins i cells. 2) We propose to develop live cell imaging using isoform-specific thioacetyllysine derivatives that can be used to visualize the dynamic activities of sirtuin isoforms in cells. We will use these tools to address several biological questions of high significance to the sirtuin field. With accomplishment of the aims, researchers will be empowered to track sirtuin activities with unprecedented specificity, to determine location, abundance and activity in live cells. These tools are predicted to accelerate studies to elucidate how sirtuins provide adaptations that improve human health.